One way to decrease the quantity of harmful species emitted in engine exhaust, such as hydrocarbons and carbon monoxide, is to better control the engine's air-to-fuel ratio during the cold start period. Such control can be improved if the volatility of the fuel is known. To obtain satisfactory engine performance and low emissions, it is also important to know the concentration of ethanol in the fuel. If the concentration of ethanol and the volatility of the fuel are known, the engine calibration can be optimized to provide satisfactory performance while controlling the hydrocarbons in the exhaust emissions.
The ethanol concentration in gasoline can be determined by measuring the capacitance between electrodes that produce an electric field in a volume that contains a sample of the fuel. The dielectric constant of gasoline fuel that contains ethanol increases with ethanol concentration. The volatility of fuel can also be found by heating a fuel sample and monitoring its evaporation as a function of temperature (or time) by measuring the capacitance between electrodes. As the sample evaporates, liquid fuel with a dielectric constant greater than about 2 is replaced by a mixture of air and fuel vapor that has a dielectric constant of approximately 1.
Sensor structures are known that can be used to measure a fuel's ethanol concentration and volatility. One approach has used closely spaced parallel plates. The gap between the plates is filled with liquid fuel while the structure is at ambient temperature by contacting the structure with liquid fuel and allowing the liquid fuel to be drawn between the plates by the capillary effect. The capacitance measured with the gap filled with liquid fuel determines the concentration of ethanol in the fuel.
To determine fuel volatility with such a structure, electrical current is passed through a heater to raise the temperature of the fuel sample. The temperature is monitored as a temperature dependent resistance. The increase in temperature causes the fuel sample to evaporate. Measured temperature and capacitance as functions of time are used to quantify the evaporation of the fuel sample. Such data are used to determine a measure of the volatility of the fuel. Driveability index (DI) is one such measure of fuel volatility.
However, devices of this type that have been demonstrated are costly to fabricate. One source of difficulty has been the effect of fuels, which are solvents, on the glues or adhesives used to join the heater, which is typically a ceramic, with the other components of the sensor. Another source of difficulty has been the effect of the repeated thermal cycling, required by the measurement, on the adhesives.